1. Field of the Invention
The present invention relates to a hydraulic pressure supply unit of a continuously variable transmission (CVT), and particularly to a friction engagement element to be engaged during the vehicle running and a hydraulic circuit used for supplying hydraulic pressure to a primary pulley.
2. Description of the Background Art
A continuously variable transmission which can continuously change a gear ratio is conventionally known. In the continuously variable transmission, for example, a metal belt or a chain is held by each of a primary pulley and a secondary pulley. Also in the continuously variable transmission, the gear ratio is changed generally by changing the hydraulic pressure supplied to the primary pulley, and more specifically, the hydraulic pressure supplied to a primary sheave of the primary pulley.
For example, as the hydraulic pressure supplied to the primary sheave of the primary pulley is increased, the groove width of the primary pulley is narrowed. Consequently, the effective diameter of the primary pulley is increased. In accordance with an increase in the effective diameter of the primary pulley, the groove width of the secondary pulley is increased and the effective diameter thereof is decreased, which leads to upshifting.
In contrast, as the hydraulic pressure supplied to the primary sheave of the primary pulley is decreased, the groove width of the primary pulley is increased. Consequently, the effective diameter of the primary pulley is decreased. In accordance with a decrease in the effective diameter of the primary pulley, the groove width of the secondary pulley is narrowed and the effective diameter thereof is increased, which leads to downshifting.
Accordingly, when the hydraulic pressure supplied to the primary sheave of the primary pulley is decreased, for example, due to some irregular conditions, unnecessary downshift may occur. Therefore, there is a need to provide the continuously variable transmission with a fail-safe function for preventing irrelevant downshifting.
Japanese Patent Laying-Open No. 2004-169895 discloses a control device of a continuously variable transmission which includes a gear ratio control solenoid valve for generating control hydraulic pressure based on an electric signal and a gear ratio control valve operated by the control hydraulic pressure to supply and discharge ATF (Automatic Transmission Fluid) to and from a drive pulley, and changes the gear ratio by the relative change between the primary pressure acting on the drive pulley and the line pressure acting on the driven pulley. The gear ratio control valve is configured so as to allow the ATF to be supplied to the drive pulley in the state where the electric signal cannot be conducted to the gear ratio control solenoid valve. Between the gear ratio control valve and the drive pulley, a pressure-adjusting valve is provided for maintaining a prescribed primary pressure by supplying and discharging the ATF to and from the drive pulley by the feedback pressure from the drive pulley, in the state where the electric signal cannot be conducted to the gear ratio control solenoid valve.
According to the control device of the continuously variable transmission disclosed in Japanese Patent Laying-Open No. 2004-169895, in the case where the ATF leaks, the feedback pressure falls and the pressure-adjusting valve brings the drive pulley into communication with the gear ratio control valve, which allows the ATF to be supplied to the drive pulley. Accordingly, the primary pressure can be maintained at the prescribed pressure, with the result that the gear ratio can be maintained.
However, the valve having only a fail-safe function as disclosed in Japanese Patent Laying-Open No. 2004-169895 is additionally provided, which poses a problem that the number of components increases.